> Why is this labelled "fail"? This clearly is an intentional joke, an not a half-bad one either.

This is true. However, the labelling of "FAIL" is an attempt to go along with the joke; it is expected not only of the viewer to recognize the sign as an intentional joke, but the labelling of the photograph as a joke as well.

This is called "dead-pan" (with "pan" being a slang term for the face or the expression) humor.